This invention relates to civil engineering and construction machinery, and more particularly, to a hydraulic drive system for civil engineering and construction machinery, such as, for example, hydraulic shovels, hydraulic cranes, etc., with the drive system comprising at least one hydraulic pump driven by a prime mover, at least one actuator connected to the hydraulic pump and at least one hydraulic cylinder, wherein the hydraulic actuator and the hydraulic cylinder are adapted to operate a plurality of movable members.
Civil engineering and construction machinery, such as hydraulic shovels, hydraulic cranes, etc., generally comprise a plurality of heavy parts, and a hydraulic drive system is usually employed for controlling these parts. For example, in a hydraulic shovel of the prior art, one hydraulic pump is used when a boom is raised at low speed and two hydraulic pumps are used when the boom is raised at higher speed, to supply pressurized liquid to the bottom side of a boom cylinder thereby controlling the speed at which the boom is raised. When the boom is lowered, a directional control valve is actuated to return the liquid from the bottom side of the boom cylinder. In this case, the lowering of the boom takes place at high speed due to the potential energy of the boom if one directional control valve is brought to a full open position. To prevent the boom from being lowered too fast, it has been customary to control the speed of downward movement of the boom by bringing the directional control valve to an intermediate position thereby throttling the flow of liquid through a line connecting the bottom side of the boom cylinder to a liquid reservoir. When it is impossible to obtain enough speed for lowering the boom merely by bringing one directional control valve to a fully open position, two directional control valves are employed to return the liquid from the bottom side of the boom cylinder to the reservoir.
The boom of a hydraulic shovel being very heavy by itself, it has potential energy of very high magnitude when raised to a high position. However, in hydraulic drive systems of the prior art, it has been customary to control the speed of downward movement of the boom by throttling the flow of liquid through the line from the bottom side of the boom cylinder to the reservoir by actuating a directional control valve to allow the potential energy of high magnitude to be dissipated in the form of thermal energy. Thus, the hydraulic drive systems of the prior art have suffered the disadvantage that no full utilization of energy can be realized with high efficiency.